This project will develop functional magnetic resonance imaging (FMRI) methods for diagnosis and monitoring the progression of neuromuscular diseases and other fatigue syndromes. Aim 1 of the project will develop a non-invasive FMRI method for the quantitative assessment of motor unit loss in the muscles of patients with compensated peripheral muscle denervation. This aim is based on previous results, which indicate that the heterogeneity of the metabolically-linked increase in transverse relaxation time (T2) within exercised muscles depends on the number, size, and spatial distribution of motor unit territories in the muscle. This new idea will be tested by comparing indices of T2 heterogeneity (e.g., T2 variance, T2 spatial autocorrelation coefficient) after exercise in anterior tibialis muscles of well-compensated Amyotrophic Lateral Sclerosis (ALS) patients vs, age and sex matched control subjects (20/group). FMRI results will be correlated with EMG based motor unit number estimates in both groups. In addition, a subset of eight ALS patients will be examined by both MRI and EMG after one year, in order to examine the ability of FMRI to detect loss of motor units in individual patients during disease progression. Finally, a separate study will examine if the same FMRI procedures can be used to measure the age-dependent loss of motor units in otherwise healthy elderly vs. younger subjects (12/group). Aim 2 of the project will examine the physiologic basis of a new variant of muscle FMRI based on transient changes in muscle oxygenation and blood flow after brief contractions. Analogous blood-oxygen-level-dependent (BOLD) effects are commonly exploited in brain FMRI studies, but not in muscle studies, which have been based on the T2 increase during more intense exercise. First, this Aim will examine the physiologic basis of muscle BOLD FMRI by comparing transient BOLD changes with transient changes in vessel blood flow (MRI and Doppler ultrasound) and hemoglobin saturation (near infrared spectroscopy, NIRS) in healthy subjects. The results will be compared to the BOLD responses predicted from models developed by others to explain the analogous BOLD effects in the brain. Second, this Aim will explore the application of BOLD-based muscle FMRI for quantitative imaging of muscle vascular reactivity by comparing MRI and NIRS results in healthy elderly vs. young subjects. This study will utilize the same elderly and young subjects as Aim 1, enabling direct comparison of BOLD vs. conventional T2-based muscle FMRI. Aim 3 will examine the quantitative relationship between muscle recruitment and fatigue vs. the intensity of BOLD FMRI response in motor cortical areas in the brain. This study of healthy young subjects will resolve two questions that are essential for any future application of brain FMRI to study central involvement in various fatigue syndromes: first, is the intensity of the BOLD response in motor cortical areas quantitatively dependent on force development, and second, how is this central response altered by peripheral muscle fatigue?